Microwave electronic amplifier



Dec. 26, 1939. E. G. LINDER 2,184,556

MICROWAVE ELECTRONIC AMPLIFIER Filed July 25, 1936 nventor Patented Dec. 26, 1939 Para lWIOROWAVE ELECTRONIC AMPLIFER Ernest G. Linder, Philadelphia, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application July 25,

8 Claims.

This invention relates to micro-waveampliiers. lMore speciiically, it relates to improved magnetrons or electronic discharge devices which are provided with means for obtaining an operating characteristic and means suitable for amplifying ultra high frequency currents.

Numerous limitations in present day triode and pentode thermionic amplifying tubes render these tubes practically useless as amplifiers of micro-waves. For example, in a conventional triode, a very small input capacitance will act as a substantial short circuit at, frequencies of the order or" several thousand megacycles. Ii magnetrons or positive grid ltubes are used, the capacity of the electrodes may be minimizedand made part of the resonant circuit. I have devised improved means and adjustments for operating the latter types of tubes as micro-wave amplifiers.

One of the objects of my invention is to provide an improved method of and means for operating a magnetron as an amplifier of ultra high frequency currents. Another object is to provide animproved method of and means for operating a positive grid electronic discharge tube as a microwave amplifier'.v

My invention may be best understood by referring to the accompanying drawing in which Fig. 1 is a schematic diagram o-f magnetron embodying my invention,

Fig. 2 is a schematic diagram of an .embodiment of my invention applied to -a positive grid tube, and

Figs. 3 and 4 are graphs illustrating one mode of operation of the circuit shown in Fig. 1.

Referring to Fig. l a magnetron I is composed of an evacuated envelope 3 within which` are suitably mounted a cathode 5, a pair of split anodes 'l and a pair of end plates 9. A magnetic field is created by a solenoid il, or thelike, which surrounds the envelope 3. 'Ihe solenoid may be energized by a battery I3. 'Ihe anodes l are connected to a transmission line l5. The transmission line l5 terminates in a dipole antenna Il which may be moved along the transmission line to resonate or tune the line. The anode power supply i9 is connected between the cathode 'l and the midpoint of the dipole il through the radio frequency choke coil 2l. The end plates 9 are made positive by a connection to a source of potential such as the anode power supply. While the end plates are not essential, I prefer to use them as they aid in making adjustments oi the tube characteristics. The cathode 5 is energized by any suitable power source 23. The output of the amplier is represented bythe loop 193s, serai N0. 92,561 (o1. 1794-171) 25 which is coupled to the transmission line and terminated in a detector, cr the like, not shown.

Having described the circuit, I shall briefly de,- scribe the operation and method of adjusting the tube operating characteristlcs. Electrons emitted from the cathode "i, if no magnetic eld be applied, travel in straight lines to the anodes. As a magnetic eld,-whose lines of force are substantially parallel to and surround the cathode, is applied, the path of the electrons becomes curved. As the density of the magnetic field increases, radius of curvature of the electron path decreases. The magnetic field density may be made suiclent to return all the electrons to the cathode; i. e., cut off the anode current.

The vario-us curvatures, effected by the mag.- netic eld, establish diierent electronv transit times, which may be related to the oscillatory frequency or wave length. This relation between wave length, and magnetic field is as follows:

where k=wave length in centimeters, and H=magnetic eld in gauss. yThe applied eld may also be related to the anode potential and anode radius as follows:

lli' where H=eld in gauss, V=anode potential in volts', and r=anode radius in centimeters.

'Ihe frequency or Wave length of the amplier is adjusted to the transmitter wave-length by resonating the transmission line. The next step is to adjust the magnetic field, in accordance with so that the electron transittime will be substantially equal to the period of the transmitted wave. The anode potential may now be adjusted until the electrons just graze the anodes, i. e.. to just establish a small anode current. The anode current may be adjusted by varying the electron emission or by suitably adjusting the end plate current. Assuming no incoming waves are being received, the characteristic may be represented by curve A of Fig.v 3. This curve was obtained by plotting the relative radio frequency output against various values of D. C. anode current. For example, the output power was measured in loop 25, although current or voltage measurements would be saisactory. It will be observed that the radio frequency output is substantially zero until theanode current reaches .16 milliampere and the tube begins to oscillate. Above .16 milliampere the output current and the anode current both increase and the device is generating oscillatory currents.

If the transmitted waves are impressed on the resonant transmission line, the output will change from the characteristic A to B. Curve B is obtained by adjusting the tube to a given value of anode current when no signal is present, and measuring the radio frequency output when a signal is impressed on the antenna. For low values of anode current, the received currents trigger the receiver into an oscillatory state. For high values of anode current, the oscillatory output is shifted by the incoming signal as shown. The difference between the curves A and B is plotted in Fig. 4. The curve C of this figure represents the relative change in the radio frequency output caused by the application of the signal. It will be observed that a maximum positive increase D, or a maximum negative decrease E may be obtained by regulating the anode current to .18 or .21 milliampere, respectively, by varying the cathode emission, end plate current or both.

The relative increase or decrease in output a measure of the gain of the amplifier. Amplification of the carrier frequency of the incoming signal takes place at any point on curve C which is above the Zero ordinate of output. Thus, when the tube draws an anode current corresponding to a point on the curve above the zero ordinate, and when a modulated carrier frequency is impressed on the tube, an ampliiied modulated carrier of like frequency will appear at the output. When the tube is operated with an anode current corresponding to a point on the curve below the Zero ordinate, a signal tends to decrease the output. In this case, the amplification of the signal carrier is less than unity. But changes in carrier amplitude, produced by modulation for example, give rise to amplified changes in the output. That is, the effective; percent modulation of the output signal has been increased, even though the maximum carrier intensity has been decreased. In either case, changes in the amplitude of the incoming signal result in changes in the output of the amplifier whose direction and degree is indicated by curve C.

The maximum amplification of the carrier amplitude is obtained at point D on curve C. The maximum resultant change in output produced by changes in the amplitude of an input signal is obtained at point E. The frequency of the output current substantially equals the incoming carrier frequency. The amplified currents are impressed on the resonant output circuit which is coupled to the transmission line.

The circuit arrangement shown in Fig. 2 is similar to Fig. 1 but differs from the latter by the substitution of a positive grid electron tube for the magnetron. In Fig. 2 a positive grid tube is represented by reference numeral 21. The tube is composed of an evacuated envelope within which are concentrically mounted a cathode 29, grid 3l, and split cylindrical anodes 33. The cathode 29 is heated by a battery 35. The grid 3| is biased positively with respect to the cathode by a battery 3l. The anodes 33 may be biased slightly positive or negative by a battery 39 which is connected from the cathode 3l through a radio frequency choke coil lll to the midpoint of a dipole antenna 43. The dipole 43 is adjustably connected to a transmission line 45 which is terminated in the split anodes 33. The Work or output circuit is composed of a loop lll which is coupled to the transmission line 45. The output circuit may include additional amplifiers, or a detector. f

The operation of a positive grid electron tube is essentially as follows: The electrons move along radial lines from the cathode through the grid to or toward the anode. Some of the electrons are then repelled by the anode and move toward the grid. Under the controlling electrical forces of a tuned circuit a cloud of electrons oscillate back and forth between anode and grid. The transit period of the electrons may be adjusted to substantially the period of the tuned transmission line Which connects the anodes. The operating characteristics of the positive grid tube may be adjusted to give a characteristic similar to the magnetron operation illustrated by Figs. 3 and 4.

The precise theory of operation of the electronic amplifiers is not entirely understood but it is believed that both the magnetron and positive grid amplifiers operate substantially as follows: The electrons, traveling their cyclic paths, are timed so that their transit periods substantially equal the frequency of the incoming carriers. The incoming carrier currents are resonated by the tuned transmission line. The effect of the resonated incoming signal currents is to substantially increase the movement of the electrons. The increased electron movement is accompanied by changing space charge conditions and changing anode current. The resulting changes, affect the output to thereby produce amplified currents which may be transferred to the output or work circuit.

I claim as my invention:

l. A microwave amplifier comprising in combination a magnetron amplifier having cathode and anode electrodes, a tunable circuit connected to said electrodes, means for impressing a magnetic eld on said magnetron, means for impressing incoming carrier currents on said tunable circuit, and an output circuit coupled to said tunable circuit at a point of R. F. potential, whereby amplified currents substantially of the frequency of said carrier currents are available at said output circuit.

2. A microwave electronic amplifier comprising in combination an electron discharge tube having cathode and anode electrodes, a resonant transmission line connected to said electrodes, means for impressing incoming carrier currents on said transmission line, an output circuit substantially resonant to the frequency of said carrier currents coupled to a point of radio frequency pctential on said transmission line, and means `for adjusting the operating characteristics of said tube so that said incoming carrier currents effect increased output currents substantially of the frequency of said carrier currents in said output circuit.

' 3. A microwave electronic amplifier comprising in combination a magnetron tube having cathode and anode electrodes, a resonant transmission line connected to said electrodes, means for creating a magnetic field for said magnetron, means for impressing incoming carrier currents on said transmission line, an output circuit coupled to a point of radio frequency potential on said transmission line, and means for adjusting the operating characteristics of said tube so that variations in said incoming carrier currents effect increased variations in the output currents in said output circuit, said output currents having substantially the same frequency as said incoming `carrier currents.

4. A microwave electronic' amplifier comprising in combination a magnetron tube having cathode, end plates and anode electrodes, a resonant transmission line connected to said electrodes, means for creating a magnetic eld for said magnetron, means for impressing incoming carrier currents on said transmission line, an output circuit coupled to said transmission line, and means including said end plates for adjusting the operating characteristics of said tube so that said incoming carrier currents effectincreased output currents in said output circuit.

5. A microwave electronic amplifier comprising in combination a positive gridy electronic tube having cathode, grid and anode electrodes, a resonant transmission line connected to said anode electrodes, means for impressing incoming carrier'currents on said transmissionline, an output circuit coupled to a point of radio frequency potential on said transmission line, and means including said positive grid for adjusting the operating characteristics of said tube so that said changes in said incoming carrier currents effect increased Achanges in said output currents in said output circuit.

6. The method of amplifying microwaves by means of a magnetron tube which comprises applying a magnetic eld to said magnetron so that the electron transit timesubstantially equals the period of said microwaves, resonatingsaid microwave currents, impressing said resonated currents on said magnetron tube, obtaining output currents having a frequency substantially equal to that of said microwave currents, and adjusting the anode current in said magnetron tube so that the amplified changes in said output currents are effected by changes in said resonated currents.

7. The method of amplifying microwaves by means of a magnetron tube having cathode, .end plate and anode electrodes which comprises applying a magnetic field to said magnetron so that the electron transit time substantially equals the period of said microwaves, resonating said microwave currents, impressing said resonated currents on said magnetron tube, and adjusting the endplate current infsaid magnetron tube' sothat the output currents in said tube whose frequency is substantially equal to that of said microwave currents are 'substantially increased by the appiication of said resonated currents.

8. A microwave electronic amplifier comprising in combination an electron discharge tube having cathode and anode electrodes, a resonant transmissionline connected to said electrodes, means for impressing incoming carrier currents on said transmission line, an output circuitsubstantially resonant to the frequency of said carrier currents coupled to a, point of radio frequency' potential on said transmission line, and means for adjusting the operating characteristic-s of saidrtube so that variations in the amplitude of said carrier currents effect increased variations in the amplitude of the output currents in said output 'circuit, said output currents having substantially vthe same frequency as said carrier currents.

ERNEST G LmDEa 

